Chapter 10: How Galaxies Grew

Night 110: A Lesson With Water Drops

Day 110 was a Saturday on the station's clock. Weekends in orbit are not completely free, because some jobs never stop, but they are lighter. In the morning, the crew cleaned the station together. They wiped surfaces, changed air filters and vacuumed the grilles where dust, crumbs and small lost objects collect, carried there by the moving air. Ade found a screw that someone had lost weeks ago, and Tomás declared it the find of the day.

Weekends also bring a little time of one's own. Tomás spent an hour with his music, playing quietly to himself with the headphones on. Ade read a book on his tablet with his feet hooked under a rail. Each of them had a private call with family planned for the evening. A space station is a workplace, but on a Saturday it becomes, for a few hours, something closer to a home.

In the afternoon, Noor had a task that she had been looking forward to. The ground had asked her to film a short lesson for schools. Children had sent in questions about life on the station, and some of the questions were about water. What happens to water when nothing pulls it down? Does it pour? Does it splash? Does it fall?

Noor set up a camera in a quiet corner and squeezed a little water out of a drinking pouch. It did not pour. It came out as a wobbling blob and then, slowly, pulled itself into a nearly perfect ball, floating in the air in front of her. She touched it gently with a straw, and it shivered all over and became round again. She added a second, smaller drop, and when the two touched, they joined at once into one larger ball.

She explained for the camera that water pulls on itself. At its surface, the water is always pulling inwards, a little like a stretched skin. On the Earth, gravity is stronger than this pull, so water spreads out in a puddle or runs downhill. On the station, in weightlessness, the water's own pull wins, and the shape that holds the most water with the smallest surface is a ball. At the end, she caught the drops carefully with a towel, because loose water near electrical equipment is a real risk.

That evening, she sent Sami a short piece of the film. He wrote back that his daughter, who was seven, had watched it three times and now wanted to be an astronaut, or possibly a water drop. Then he asked something that had been on his mind since the lesson. If small drops join into bigger ones, is that how galaxies were made too? Noor thought about it. In a way, yes, she wrote. But the force that brings galaxies together is not the pull of a water surface. It is gravity. And part of it comes from something nobody can see.

From Ripples to a Web

On the previous night, we saw the first stars switch on in the darkness. They formed in clumps of gas, in the places where the young universe was slightly denser than average. Tonight's story is about what happened next: how those first small clumps grew, joined together and became the galaxies that fill the universe today, including the Milky Way, our own galaxy.

The story begins with the tiny ripples that we saw in the oldest light, differences of about one part in 100,000. Gravity turned these small differences into large ones. A place that is a little denser pulls a little harder, gathers more matter and becomes denser still. Over hundreds of millions and then billions of years, the ripples grew into clumps, and the clumps grew into larger structures.

There is a reason why gravity, of all the forces of nature, was able to do this work. In daily life, gravity is very weak. A small magnet can lift a paper clip against the pull of the whole Earth. But gravity has one special quality: it only pulls and never pushes, and it adds up. Electric charges come in two kinds, positive and negative, and in large amounts of matter they mostly cancel each other out. Gravity never cancels. Over the huge distances and long times of the universe, it becomes the force that shapes everything.

When astronomers map where galaxies are found today, they see a remarkable pattern. Galaxies are not scattered evenly, like sugar sprinkled on a table. They are arranged in long threads and thin walls, with huge, nearly empty regions between them. Where threads meet, there are crowded groups and clusters of galaxies, some containing hundreds or thousands of galaxies. Astronomers call this pattern the cosmic web.

If you have ever seen a spider's web covered with morning dew, or the foam on a cup of coffee when the bubbles have pressed together, you have seen something with a similar look: thin edges and joins, with open spaces between them. The cosmic web is, of course, enormously larger. A single empty region in it can be hundreds of millions of light-years across. But the idea is the same. Matter has gathered along lines and at meeting points, and it has drained away from the spaces in between.

Computer models of the universe, which start with the ripples measured in the oldest light and then follow gravity forward through billions of years, produce a web that looks very much like the real one. That agreement is one of the reasons why astronomers think that they understand the broad story of how the structure of the universe grew. But the models only work well if they include one very important ingredient, something that nobody has ever seen directly.

The People Who Noticed Something Missing

The first clue came in 1933, from an astronomer named Fritz Zwicky. He was studying a large cluster of galaxies in the constellation called Coma Berenices. He measured how fast the galaxies in the cluster were moving. Galaxies in a cluster move around each other, held together by their combined gravity, like people moving around a crowded room who never quite leave it. The faster they move, the more gravity is needed to hold them together.

Zwicky found that the galaxies were moving far too fast. If the cluster contained only the matter that astronomers could see, in the form of shining stars, it should have flown apart long ago. Yet there it was. Something must be adding a great deal of extra gravity. Zwicky wrote of "dark matter", matter that does not shine but does pull. Many astronomers of the time were not convinced. The measurements were difficult, and it was possible that something had been overlooked. The idea waited.

Decades later, in the 1970s, the astronomer Vera Rubin studied how stars and gas move around inside spiral galaxies. A spiral galaxy turns slowly, like a great wheel. You might expect it to turn like our solar system, where the planets far from the Sun move more slowly than the planets close to it, because the Sun's pull is weaker far away. Most of the light of a galaxy comes from its bright centre, so astronomers expected the outer stars to move slowly too.

That is not what Rubin found. Galaxy after galaxy, the stars and gas far out at the edges were moving just as fast as those closer in. They were moving so fast that they should have been flung away, unless there was much more matter in the galaxy than could be seen, spread out far beyond the bright stars. Rubin was careful and patient, and her measurements were hard to argue with. Together with the work of others, they convinced most astronomers that dark matter is real. It was a good example of steady, careful work changing what a whole field believes.

Since then, other evidence has been added. Mass bends the path of light that passes near it, and astronomers can see background galaxies whose light has been bent and distorted by clusters in front of them. The amount of bending shows how much mass is there, and again it is far more than the visible matter. The patterns in the oldest light, too, can only be explained if there is a large amount of dark matter. Today, astronomers estimate that there is about five times more dark matter than ordinary matter in the universe.

What We Do and Do Not Know

So what is dark matter? The honest answer is that nobody knows. We know that it has gravity, because we can see its pull. We know that it does not shine, does not reflect light and does not block light, because otherwise we would see it. We know that it does not seem to bump into ordinary matter in any way that we have been able to notice. Beyond that, its nature is one of the biggest unanswered questions in science.

If dark matter is made of particles, as many scientists think, then those particles are probably not only far away in other galaxies. They may be all around us, here in the Milky Way, passing quietly through the Earth, through the station and through your room as you read, without touching anything. This is not certain, because we do not know what dark matter is. But it is a strange and gentle thought: the most common kind of matter in the universe may be passing through you right now, and you would never feel it.

Some things have been mostly ruled out. Could dark matter simply be ordinary matter that is hard to see, such as cold gas, dust, dim stars, dead stars or black holes? Astronomers have searched for such objects, and they do find some, but not nearly enough. There is also a deeper reason. The first few minutes, which we met on an earlier night, set the total amount of ordinary matter in the universe, and the oldest light confirms that amount. It is far too little to explain all the extra gravity. So most of the dark matter must be something different from the atoms that make up stars, planets and people.

Many scientists think that dark matter is a kind of particle that has not yet been discovered. Around the world, scientists have built very sensitive detectors, often deep underground where they are shielded from other particles, to try to catch a dark matter particle bumping into ordinary matter. Others search for signs of dark matter in particle accelerators or in the sky. So far, none of these searches has found it. That does not mean that it is not there. It means that it is harder to catch than people hoped, or different from what they expected.

A small number of scientists have tried a different idea: perhaps there is no extra matter at all, and instead gravity behaves differently over very large distances than our theories say. This idea has had some successes in explaining how galaxies spin. But it has had great difficulty explaining the clusters of galaxies, the bending of light and the patterns of the oldest light all at once, which dark matter explains together. For now, most astronomers think that dark matter is the better explanation. But the question remains open, and that is exactly how it should be until the evidence settles it.

Whatever it turns out to be, dark matter played a key role in the growth of galaxies. Ordinary matter, in the young universe, was pushed around by light, and it could not gather into clumps very easily until the oldest light was released. Dark matter does not feel light at all. So it could start gathering into clumps earlier. When the ordinary gas was finally free, it fell into the clumps of dark matter that were already waiting. Without that head start, galaxies would have taken much longer to form, and the universe today might look very different.

Galaxies That Join Together

The first galaxies were small. As we saw on the previous night, the Webb telescope sees galaxies from a few hundred million years after the beginning, and they are much smaller than the Milky Way. How did large galaxies like ours grow from such small beginnings? Partly by drawing in fresh gas from the cosmic web, and partly by joining together, like Noor's water drops, though much more slowly and much less gently.

When two galaxies come close, their gravity pulls them towards each other. They swing around each other, sometimes several times over hundreds of millions of years, and long streams of stars and gas are pulled out of them. In the end, they often settle into one larger galaxy. Astronomers have photographed many pairs of galaxies in different stages of this slow meeting. Each picture is like one frame from a film that takes far longer than a human life to play.

Our own galaxy is the result of billions of years of such growth. Today, the Milky Way is a large spiral galaxy, about 100,000 light-years across, with hundreds of billions of stars, one of which is the Sun. From the station, on a dark pass, Noor could see its band across the sky. What she saw was the inside view of a great flat disc of stars, seen edge on from somewhere within it.

The Milky Way grew partly in this way. Astronomers studying the movements of stars in our galaxy have found groups of stars that move together in unusual ways, and they think that these are the remains of smaller galaxies that the Milky Way swallowed long ago. Some of the oldest stars in our galaxy formed more than 13 billion years ago, so the Milky Way has been growing for almost the whole history of the universe. Even today, it is slowly pulling in a few small neighbouring galaxies.

And the Milky Way has a large meeting ahead. On an earlier night, we saw that the Andromeda galaxy, about 2.5 million light-years away, is one of the few galaxies moving towards us. In about 4 to 5 billion years, the Milky Way and the Andromeda galaxy are expected to come very close to each other, and over a long time they may merge into one larger galaxy. Their exact paths are still being measured, so astronomers are not yet certain when, or even whether, the merging will happen. If it does, the night sky of any planet that still exists then will be very different, full of new streams and clouds of stars.

This may sound alarming, but there is no need to worry, for two reasons. The first is the enormous time involved, far longer than human history many times over. The second is that galaxies are mostly empty space. The distances between stars are so large compared with the stars themselves that, when two galaxies merge, the stars almost never hit each other. They pass by at great distances, while the gas clouds of the two galaxies meet and may give birth to many new stars.

A Message to Sami

Before sleep, Noor watched the short film of the water drops once more, then sent the finished lesson to the ground. She thought of the children who would watch it in their classrooms, and of Sami's daughter, who wanted to be a water drop. She thought, too, of the long threads of the cosmic web, with galaxies gathered along them like drops on a spider's thread.

She wrote to Sami: "Tell your daughter that galaxies also join together, like the drops, but very slowly. Our galaxy has swallowed several smaller ones. And most of what holds galaxies together is something invisible that nobody has identified yet. Maybe she will be the one who finds out what it is." Sami answered that his daughter had fallen asleep, but that he would tell her in the morning, and that he was sure she would ask what "identified" means.

Perhaps, at the end of your own week, you also have a lighter day, with chores in the morning and something you enjoy in the afternoon. On such a day, if you pour a glass of water or watch rain gather into drops on a window, you might remember the long, slow gathering of the universe. Small things join into larger ones. Some of what holds them together is easy to see. Some of it, like many of the good things that hold people together, cannot be seen at all.

Common Misunderstandings

Many people think that dark matter is simply ordinary matter that is too dark to see, such as dust, dark clouds or dead stars. Astronomers have looked carefully for such things. They exist, but there is not nearly enough of them. The first minutes of the universe and the oldest light both show that the total amount of ordinary matter is far too small to explain the extra gravity. So most dark matter must be something else, probably something that is not made of atoms at all.

Many people also assume that scientists know what dark matter is and simply use a mysterious name for it. They do not know. They know how much of it there is, roughly where it is and how it pulls. But what it is made of has not been discovered, despite many years of careful searching. The word "dark" in its name is an honest admission. It means that we see its effects but cannot see the thing itself, and we do not yet understand it.

Finally, many people imagine that when two galaxies collide, their stars crash into each other in great explosions. In fact, stars almost never collide, even when whole galaxies merge, because the space between stars is so vast. If the Sun were the size of a grain of sand, the nearest other star would be another grain of sand more than ten kilometres away. The real meeting happens between the gas clouds, which can be squeezed and can form new stars, and those new stars, in their turn, make new elements. On the next night, we will look inside the stars and see how they made the elements of our own bodies.